Since 1990s, the wireless communication industry has experienced explosive growth. With the voice, data and video services being provided to mobile equipments, the wireless communication system needs to provide higher and higher capacity. However, available spectrum resources are increasingly scarce. Therefore, improving the spectrum efficiency becomes an essential research topic of wireless communications.
Technologies for improving the spectrum efficiency include: multiple access, signal detection, modulation and channel coding. Among such technologies, the Multi-Antenna System (MAS) is highly promising and becomes more and more important in the wireless communications.
As one of the MAS, Smart Antennas (SA) is also known as an Antenna Array System (AAS). The spacing between array elements of an SA is generally less than the correlation distance of channels. Utilizing the channel correlation of transmitted signals on SA, beam forming is implemented, and high-gain narrow beams are directed to the desired mobile terminal adaptively, and null are adjusted and directed to the interference direction.
A Multiple Input Multiple Output (MIMO) system is another style of the MAS. Foschini has proved theoretically enormous potentiality of the MIMO system for improving the spectrum efficiency. With a MIMO system, the channel capacity increases linearly with the number of antennas (in proportion to the minimum number of antennas on the transmitter side and the receiver side). A MAS can combine with Space Time Coding (STC) to generate spatial diversity, reduce the Bit Error Rate (BER) and improve the system reliability. A MAS may utilize a Bell Labs Layered Space Time (BLAST) structure, which forms parallel sub-channels, improves the capacity of the channel and supports higher data rate by means of space multiplexing. A MIMO system is similar to a SA except for different antenna spacing.
In a cellular communication system, a base station allocates a dedicated channel to each active user within a cell or sector, and the dedicated channel is designed to bear voice, data or video traffics. Based on a multi-antenna base station, beam forming or pre-coding may be applied on the dedicated channel to transmit user-specific signals, and reduce intra-cell or inter-cell interference.
In an actual mobile communication system, a cell/sector has not only dedicated channels, but also public channels. A public channel bears public information required by all mobile terminals in the cell/sector, for example, system information in the broadcast channel; reference signals in the synchronization channel; and pilot, paging and public control messages in a Forward Access Channel (FACH). The cell coverage required for a public channel is sharply different from the coverage required for a dedicated channel. In a public channel, all mobile terminals in the cell/sector need to receive signals simultaneously, and the base station needs to cover the whole cell/sector properly.
In the prior art, a solution to covering the whole cell/sector through an antenna array divides the transmitting time of the public channel into timeslots, selects a group of weighting vectors whose beam patterns complement each other, and uses the complementary weighting vectors alternately in continuous timeslots, thus covering the whole cell/sector based on SA.
In the prior art described above, although multiple complementary weighting vectors are used alternately, the total number of weighting vectors is limited, and the beam pattern corresponding to each weighting vector is fixed. Therefore, the average of the antenna gain of multiple beam patterns is not completely equal in different directions, but is approximately isotropic only. Consequently, the BER performance varies in different directions.
Besides, the prior art described above is intended for only the smart antenna system of linear arrays, and not for other MAS such as MIMO system, and not for other antenna arrays such as square arrays and circular arrays.